Open Access Te Herenga Waka-Victoria University of Wellington
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Characterising the foraging ecology and mercury exposure of the Nationally Critical Whenua Hou diving petrel

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posted on 2021-02-28, 20:37 authored by Tocker, Grace

With seabird species in decline globally, significant research has gone into characterising their key prey species and foraging areas that need protection. Knowledge on the diet of a species has important implications for the development of conservation programmes. The sand dune system on Whenua Hou is home to the endemic Whenua Hou diving petrel (Pelecanoides whenuahouensis; hereafter WHDP) and a population of common diving petrels (Pelecanoides urinatrix; hereafter CDP). The WHDP is considered ‘Nationally Critical’ due to its small population size (~200 individuals) and restricted breeding range (0.018 km2) on Whenua Hou. The foraging ecology of the WHDP is relatively unknown, as is its exposure to sources of marine pollution. This thesis aimed to characterise the foraging ecology of the WHDP, the potential interspecific competition with the sympatric CDPs, the prey present in the diets of each species, and their resulting exposure to mercury from the environment. In chapter 2, I used stable isotope analysis to infer the trophic dynamics of the WHDP. By sampling and analysing both blood and feathers, I was able to investigate potential differences in WHDP foraging ecology between the breeding and non-breeding seasons. I found a difference between the foraging ecology of male and female WHDPs, with results indicating females forage further out to sea than males and on prey of lower trophic value. I found that WHDPs forage an entire trophic level higher during the breeding season than the non-breeding season. As my sampling effort spanned three consecutive breeding seasons (2017-2019), I was able to detect interannual variation in the foraging ecology of WHDPs. The results revealed that WHDPs foraged at a higher trophic level during the breeding season of 2018 compared to that of 2017 or 2019. By characterising the isotopic niches of both the WHDPs and CDPs over the three years, I was able to demonstrate a degree of trophic segregation between the two species during the breeding season. In chapter 3, I designed and went through the initial development stages for a novel multiplex-PCR assay to identify the prey species present in the diets of WHDPs and CDPs. The obstacles faced in the development of this protocol highlighted the suitability of DNA metabarcoding as an alternative method. In chapter 4, I analysed the mercury concentration in the same blood and feather samples used for stable isotope analysis. I demonstrated that male WHDPs had higher concentrations of mercury in their tissues than females, correlating with their foraging at a higher trophic level. The interannual variation in mercury concentration did not correlate with the trophic variation of WHDPs among years, indicating that the environmental fluctuations in mercury levels had a stronger effect on mercury exposure than diet. WHDP tissues consistently had higher concentrations of mercury than CDPs, correlating with their isotopic niche segregation and highlighting a potential threat to individual survival and reproductive success in WHDPs. Overall, my results describe patterns in the foraging ecology of the WHDP, as well as highlighting the potential threat from mercury exposure. This research can be used as a baseline for future investigations into the key prey species for the endangered WHDP and the impacts mercury exposure may be having on the population growth of this species.


Copyright Date


Date of Award



Te Herenga Waka—Victoria University of Wellington

Rights License

Author Retains Copyright

Degree Discipline

Ecology and Biodiversity

Degree Grantor

Te Herenga Waka—Victoria University of Wellington

Degree Level


Degree Name

Master of Science

ANZSRC Type Of Activity code


Victoria University of Wellington Item Type

Awarded Research Masters Thesis



Victoria University of Wellington School

School of Biological Sciences


Wittmer, Heiko; Ritchie, Peter